[Java][Spring] Make generated API controller methods throwing Exception.

Spring allows general error handling based on exceptions. It requires the controller method throws plain Java exceptions. Generated API interface should not restrict this behavior. In other words the code

    @RequestMapping(value = "/users", method = RequestMethod.POST)
    ResponseEntity usersPost(@RequestBody UserCreation userCreationRequest);

should become

    @RequestMapping(value = "/users", method = RequestMethod.POST)
    ResponseEntity usersPost(@RequestBody UserCreation userCreationRequest) throws Exception;

One can make this feature configurable over the configuration parameter. For example in case of Maven plugin:

<supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate> 
<controllerThrowsExceptions>java.io.IOException,com.example.NotFoundException</controllerThrowsExceptions> 

openapi-generator version

openapi-generator-maven-plugin, version 3.3.3

Related issues/PRs

Similar request in swagger-codegen project:
https://github.com/swagger-api/swagger-codegen/issues/5686

Start by locating the Java Spring controller generation path and the Maven plugin configuration handling supporting files. Verify how generated controller method declarations are produced, then confirm that the requested exception behavior is configurable and reflected in generated output.